Agriculture Reference
In-Depth Information
Box 4.3
(continued)
The same processes of association and dissociation of H
ions, dependent on
the solution pH, occur at the surfaces of sesquioxides. The acid strength of the
surface groups changes according to the mineral's composition, so that different
minerals have different
PZC
s. For example, the
PZC
for quartz is 2-3, kaolinite
edge faces is 6-7, goethite (FeOOH) is 7-8, and gibbsite (Al(OH)
3
) is 8-9. This
means that goethite is positively charged up to ca. pH 7, and gibbsite positively
charged up to ca. pH 8. However, the
PZC
values for these minerals can change if
cations like Ca
2
or some of the trace elements, or anions like H
2
PO
4
, form
complexes with the surface (section 4.5.4). In soil, these variably charged surfaces
coexist with the constant charge surfaces of clay minerals. Indeed, positively
charged oxide coatings commonly form on negatively charged clay mineral surfaces
and provide sites for anion adsorption (fig. B4.3.2).
Figure B4.3.2
Sites for anion adsorption on negatively charged clay crystals (White 1997). Reproduced
with permission of Blackwell Science Ltd.
Diffuse layer
of cations
Sesquioxide
impurities
+
+
+
Clay crystal
The higher the bulk solution concentration, the less the outward force, hence the
more compressed the
DDL
.
The resultant of these opposing forces, which depend on ion charges and con-
centrations, determines the effective thickness of the
DDL
. For solutions of mixed
salts such as a soil solution, we use a property called the
ionic strength
(
I
) as a ge-
neric measure of ion concentration, modified by ion charge. The thickness
d
eff
of
the
DDL
can then be calculated from the approximate relationship
1
d
eff
(4.6)
I
which, from the definition of ionic strength given in box 4.4, expands to
1
d
eff
(4.7)
z
C
